Orman is running as an Independent and not a Democrat. The Democratic candidate dropped out of the race last month.

Roberts and his campaign points to Orman's past donations to Obama, Sen. Hillary Clinton and Kansas Democrats, among others, as proof that he's not as independent as he says he is. Orman said he has also donated money to Republicans, including Scott Brown in his 2010 Senate campaign in Massachusetts.

"Precisely because he was supposed to be the vote that would prevent the Affordable Care Act from becoming law," Orman said of that donation during Wednesday's debate.

Federal records show that Orman also gave money to other Republicans, including Missouri Rep. Todd Akin in 2006. Most of his recent contributions have been to Democrats.

The Kansas Senate race has tightened a great deal in recent days. Two polls released this week show Roberts leading, a shift from previous polls that have given an edge to Orman.

A CNN poll among Kansans released this week show that 68 percent of those asked disapprove of the president's performance, a key reason Roberts hopes that tying Orman to the president will help his prospects in the general election.

Roberts also had Sen. Ted Cruz campaigning on his behalf in Kansas on Thursday, hoping to get more votes from tea party supporters.